Personal care absorbent products, such as diapers, and feminine hygiene products and the like are designed to absorb body fluids. These personal care absorbent products are designed so that they transport body fluids away from the wearer of the personal care product to an absorbent core within the absorbent product and, in addition, prevent the transfer of fluids from the absorbent core back to and in contact with the wearer. To accommodate these functions, a multi-layered personal care absorbent products has been developed which is comprised of a soft, body compatible, pervious top sheet, typically a hydrophobic film, an impervious bottom sheet for retaining the body fluids within the personal care product and an absorbent core disposed between the top sheet and the bottom sheet for retaining the fluids. Fluids are transferred through the top sheet to the absorbent core where the fluids are stored until disposal of the personal care product. The impervious bottom sheet prevents the fluids stored in the absorbent core to be transferred to external surfaces.
To enhance the rate of transfer of fluid away from the wearer to the absorbent core, it has also been common practice to bond a thin, low density, cellulosic webbing to the under side of the top sheet (sometimes referred to as an acquisition/distribution layer or transfer layer) and also bond it to the absorbent core. The acquisition/distribution can be characterized as a thin, low density cellulosic web having large pore diameters. The absorbent core is designed for enhanced fluid capacity at the site of wetting with a secondary function of transporting fluid to remote areas of the absorbent core to accommodate multiple discharges of fluid. It can be characterized as a relatively thick, higher density cellulosic webbing having smaller pore diameters than the acquisition/distribution layer. Hence, it is not very effective at quickly channeling liquid away from the wearer to remote of parts therein.
Cellulosic fibers have been widely used as a component in both the acquisition/distribution layer and absorbent cores. Webs formed from untreated cellulosic fibers tend to collapse when wet thus forming a web of higher density and smaller average pore size. Webs formed from untreated cellulosic fibers also have a tendency to gather into cellulosic clumps or discrete sections. Thus, not only is the rate of fluid transfer in an acquisition/distribution layer decreased by the collapse of the untreated cellulosic fiber but also the ability of the absorbent core to transfer fluids to remote portions of the layer. To combat wet collapse of the cellulosic fibers, or alternatively to enhance compression resistance of the cellulosic fiber, it has been common practice to chemically stiffen the cellulosic fibers by treating the cellulosic fibers with chemical stiffening agents. The resulting chemically stiffened cellulosic fibers tend to act like "springs." Under pressure they resist compression and when the pressure is released these "cellulosic springs" cause the web to return to its approximate original thickness. Improved compression resistance has been found to enhance both fluid transfer and absorption.
To improve web strength formed from cellulosic fibers, it has also been common practice to incorporate crosslinkable polymeric binders to webs designed for use in acquisition/distribution layers and absorbent cores for the purpose of providing enhanced wet tensile strength. Thus, the resulting web tends to prevent clumping of the cellulosic fibers and prevent the web from undergoing separation under tension.
The following patents are provided to provide a description of wide variety of personal care absorbent product, their construction and their methods for enhancing fluid transfer and fluid retention within the personal care product.
U.S. Pat. No. 5,360,420 is representative of several patents which disclose absorbent structures incorporating chemically stiffened cellulosic fibers. Such chemically stiffened cellulosic fibers are incorporated into acquisition/distribution layers having an average dry density of about 0.3 grams/cc. More particularly, the webs are comprised of from 50% to 100% of chemically stiffened cellulosic fibers and from 0 to about 50% of a binding means for increasing physical integrity of the web, to facilitate processing and to improve end-use performance. The storage layer is comprised from about 15% by weight of a super-absorbent material and 0 to 85% of a carrier for the super-absorbent and comprised of synthetic or natural fibers. Typically cellulose fibers in the form of fluff, which include chemically stiffened cellulosic fibers, are incorporated into the web. Processing methods which can be used to form the acquisition/distribution layer and the storage layer include air-laid and wet-laid techniques.
U.S. Pat. No. 5,401,267 discloses absorbent articles which exhibit an enhanced wicking capacity. The absorbent article includes a liquid-permeable cover, liquid impermeable baffle and an absorbent. The absorbent is constructed of first, second and third members with the wicking capacity of both the first and third members being greater than the wicking capacity of the second member. The first member is described as a perforated cover sheet, and the second member is comprised of hydrophilic materials such as cellulose fibers and hydrophilic polyethylene polypropylene in an air-formed blend. The third member is typically comprised of tissue layers or cellulosic fluff.
U.S. Pat. No. 5,387,208 discloses an absorbent core for use in personal care products having improved dry-wet integrity. The absorbent core is comprised of an absorbent means such a crepe cellulose wadding, melt-blown polymers including coform; chemically stiffened, modified or cross-linked cellulosic fibers, and the like. The absorbent core may also have caliper-zones, hydrophilic gradients and also incorporate super absorbent gelling materials.
U.S. Pat. No. 5,137,537 discloses absorbent structures for use in personal care products containing individualized, cross-linked fibers. The cross-linked fibers are described as being useful for producing lower-density absorbent products. A wide variety of chemical cross-linking agents are suggested for curing the individualized cross-linked fibers and these include aliphatic and alicyclic C.sub.2 -C.sub.9 polycarboxylic acids, glyoxal and so forth.
U.S. Pat. No. 5,460,622 discloses absorbent articles such diapers, sanitary napkins, adult incontinence devices and the like. The absorbent articles are comprised of blends of different types of fibers for providing improved integrity and liquid processing capabilities. More particularly, the absorbent core comprises a blend of cellulosic fibers, absorbent gelling material and crimped synthetic fibers, the function of the crimped synthetic fibers being to improve integrity, acquisition rate, absorbent capacity and resilience of the acquisition layer. The synthetic fibers are described as crimped polyester fibers which are not affected by the presence of moisture and therefore, do not collapse as do cellulosic fibers when wet.
U.S. Pat. No. 5,522,810 discloses a compression resistant and resilient nonwoven web made up of randomly-deposited fibers bonded to one another by one or more bonding methods, such as air laying, spunbonding, and bonded carded web formation. In order to obtain a compression resistant web, at least a portion of the fibers forming the web should be made from polymers which are heat bondable such as polyolefins, polyesters, polyamides, and polyvinyl alcohol. The resultant product is typically used as a top sheet or as a separation layer in personal care absorbent products.
U.S. Pat. No. 5,190,563 discloses a process for making individualized, crosslinked fibers by contacting the fibers with a solution containing a C.sub.2 -C.sub.9 polycarboxylic acid, such as citric acid, separating the fibers into individual form, drying the fibers, and then reacting the crosslinking agent with the fibers to form intrafiber crosslink bonds. The product is reported to exhibit improved absorbency and increased wet resilience compared to absorbent cores made from conventional, uncrosslinked fibers or prior known crosslinked fibers.
U.S. Pat. No. 5,104,923 discloses the combination of a crosslinkable binder and a polycarboxylate catalyst for imparting high wet strength to nonwoven cellulosic materials. The binder is formed from an aqueous emulsion polymer such as styrene-butadiene-itaconic acid copolymer. Representative nonpolymerizable catalysts include sodium ethylene diamine tetraacetate, citric acid and oxalic acid it can be incorporated in an amount of from 0.1 to 3%.